An air bag assembly unit is provided for an occupant restraint systems in motor vehicles with an air bag, which is accommodated by an air bag housing and is to be filled with gas by the contents of a gas generator. The assembly unit includes a diffusor housing, which is arranged at the air bag housing, is connected to the air bag housing via gas passage openings, and in which the gas generator pushed in via an installation opening is fixed by a holding device. The holding device is formed from a fastening ring (7), which is arranged in the intermediate space (5) of the diffusor housing wall (8) and the gas generator wall (9), seals the intermediate space (5) by a sealing device (6) in a gas-tight manner and surrounds the gas generator (2) in a ring-shaped pattern. The fastening ring (7) is fixed at the diffusor housing wall (8) in a positive-locking manner.